Automobiles and other transport equipment are demanded to have lighter self weights so as to achieve lower fuel consumption thereof. The lighter weights are effectively achieved typically by reducing the thickness of the equipment using high-strength steel sheets. In addition, the automobiles intensively require crashworthiness. To achieve this, structural parts such as pillars, and reinforcing parts such as bumpers and impact beams require still higher strengths. However, steel sheets, when designed to have higher strengths, have inferior ductility and offer poor workability. The high-strength steel sheets therefore require strength and workability both at satisfactory levels (satisfactory TS×EL balance). Most of automobile steel parts employ steel sheets applied with a zinc coating such as hot-dip galvanized coating (GI), hot-dip galvannealed coating (GA), and electrogalvanized coating (EG) from the viewpoint of corrosion resistance. These steel sheets are hereinafter also representatively referred to as “galvanized steel sheet(s)”. For the above reason, the galvanized steel sheets also require properties as with the high-strength steel sheets.
As techniques to provide high-strength steel sheets having strength and workability both at satisfactory levels, some literature proposes as follows. Typically, Patent literature 1 proposes a high-strength steel sheet. This steel sheet includes a microstructure containing a ferrite matrix, and second-phase martensite and retained austenite dispersed in specific proportions in the matrix and has excellent elongation and stretch flangeability.
Patent literature 2 proposes a high-strength cold-rolled steel sheet. This steel sheet has controlled Si and Mn contents, includes a microstructure mainly containing martensite and ferrite and further containing retained austenite, and offers excellent coating adhesion and ductility.
Patent literature 3 proposes a high-strength cold-rolled steel sheet. This steel sheet includes a microstructure containing ferrite, tempered martensite, martensite, and retained austenite and offers excellent workability and shock resistance.
Patent literature 4 proposes a high-strength steel sheet. This steel sheet includes a microstructure containing bainitic ferrite, martensite, and retained austenite, offers excellent ductility and stretch flangeability, and has a high tensile strength of 980 MPa or more.
In particular, recent steel sheets typically for automobiles require not only the proposed strength and workability, but also better safety in an assumed use environment. Typically, the steel sheets are desired to have excellent low-temperature toughness as their properties on the assumption of body collision under low-temperature conditions in the winter. However, such steel sheets, when designed to have higher strengths, tend to have inferior low-temperature toughness. This unfortunately causes the conventional steel sheets to hardly ensure sufficient low-temperature toughness, where the conventional steel sheets are provided so as to have higher strengths and better workability. Demands have therefore been made to provide steel sheets having still better low-temperature toughness.